Use of the Barkhausen effect in the measurement of residual stresses in steel

For on-line monitoring of the state of materials, various non-destructive m ethods exist, among them the micro-magnetic method with which a ferromagnet ic material is magnetised by an alternating current while the Barkhausen no ise (BN) generated is monitored. A suitable experimental set-up was compose d. The paper deals with determination of residual stresses in samples of he at-treatment steel C45E which were hardened and tempered at three different temperatures and eventually flat ground. By means of a measuring coil of H ull probe, the changes in the induced voltage in magnetisation, which are d ue to various influences of the analysed material, were detected. Further p rocessing of the captured voltage signals was carried out on samples being in different states, and calibration curves were determined. A final decisi on on the state of the material was obtained from the calibration curves. O n the basis of the micro-magnetic parameter obtained, the parameter of surf ace integrity searched for can be determined. The assessment of the efficie ncy of determination of residual stresses by the micr-magnetic method based on the BN was carried out by means of measurement of residual stresses wit h the relaxation method. Comparative through-thickness measurements showed that the micro-magnetic method was suitable for determination of residual s tresses since it gives a real-time variation of residual stresses in a thin surface layer of the material concerned in a very short time.